barbanegra.
The advice given here is very sound. I went through exactly what you are going through my two boys are young.
Don't move out until you are ready to move out, if ever. But as said, avoid confrontation. You don't want to give her 'cause.'
Have you started the divorce action? If not, I would recommend you go for Unreasonable Behaviour on grounds of Inappropriate Relationship. If she is actually telling your son about her new boyfriend then I would add an allegation of 'emotional abuse towards you and the child' by discussing matters of an adult nature with the child. Don't waste time and get onto it right away if you haven't done it yet. If you have not got hold of the marriage certificate, do that now, right now, you will need it.
Try to keep your kid out of it.
Write a polite and short note back to her solicitor saying that you do not intend to leave the former
matrimonial home at this time. If there are any accusations towards you in the letter state that you deny the accusations, don't go into detail. The less said the better.
Do not give your consent regarding the new relationship. If asked say it is inappropriate, nothing more.
If you have joint accounts either freeze them or take the money out and put it into an account in your sole name before she does this to you. Don't withhold money, just take control of it.
Some might disagree with the very direct approach I am stating, but she will not be in a rational mindset if she is in the throws of a new romance and things will get worse from here so it is better to at least be in control of the divorce and money (cashflow only, the financial settlement is separate and you can't 'control' that item.)